Finding Black Holes with Microlensing
Abstract
The MACHO and OGLE collaborations have argued that the three longest-duration bulge microlensing events are likely caused by nearby black holes, given the small velocities measured with microlensing parallax and non-detection of the lenses. However, these events may be due to lensing by more numerous lower-mass stars at greater distances. We find a-posteriori probabilities of 20%, 8%, and 3% that the three longest events are black holes, assuming a Salpeter initial mass function and a 40 solar mass cutoff for the neutron-star-progenitor mass; the detailed results depend strongly on the assumed mass function but do not favor black holes for any standard IMF. The longest-timescale events (>600 days) have an a-priori ~26% probability to be stellar remnant black holes for a Salpeter IMF. We propose a new means for measuring the lens mass function using the mass distribution of long events measured with ACS, VLTI,or SIM.Keywords
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